This invention pertains to high speed composite flywheel systems, and more particularly to a composite flywheel rim having a thin metal coating to prevent off-gassing from the composite resin during operation in the flywheel vacuum chamber.
Flywheel systems have been used for many years for storing energy in the systems, and then releasing that stored energy back into the other system. They provide a smoothing effect for internal combustion engines and many kinds of power equipment. More recently, modern flywheel systems have become recognized as very attractive energy storage systems in electrical applications such as uninterruptible power supplies, utility load leveling, electric vehicles and battery replacement.
Modern flywheel systems used for electrical energy storage convert the electrical energy to mechanical energy of a high speed rotating flywheel rotor, and back again. The flywheel system includes a flywheel rotor (hub and rim), and stator on the hub shaft that function as an electric motor during storage of electrical energy and as a generator during regeneration of electrical energy when the stored energy is to be recovered. The flywheel system is normally contained in a vacuum enclosure that protects it from windage losses that would occur from operation in a gas atmosphere, and provides ballistic protection against possibly catastrophic failure of a flywheel rotor rotating at high speed.
The flywheel rim usually comprises a composite ring made of resin-impregnated filaments wound in the hoop direction. The rim can be made of one or multiple types of fiber in a concentric ring arrangement. One example would be using an E-glass/epoxy ring inside a carbon/epoxy ring. The fibers are typically wound on a mandrel as B-staged prepreg or wet rovings, and then are cured at moderate temperature in a curing oven or autoclave. During curing, the resin outgasses normally and the gasses and vapors are evacuated through the usual ventilation system in the curing facility.
After curing, the flywheel rim appears to be stable, but in fact the resin continues to outgas at a very low rate. These outgasses include volatiles in the resin and water in the resin and adsorbed in the surface of the fibers. Ordinarily, these gasses and vapors are insignificant because they are at such a low rate that they are virtually undetectable. However, in a vacuum, which is the environment in which a flywheel operates, the rate of outgassing from a cured resin in a composite flywheel rim is both greater and of more significant consequence. The rate is greater because the rate of evaporation is a function of partial pressure, which is low in a vacuum chamber, and the consequence is greater because the vacuum in the vacuum chamber becomes degraded by the outgassing. Degraded vacuum results in increased windage losses for the flywheel rotor and increased energy costs in maintaining the flywheel rotor at the desired operating speed.
Composite material flywheel rims which rotate at high speeds, sometimes with peripheral speeds in excess of three times the speed of sound, typically operate in an evacuated chamber. The use of an evacuated chamber prevents aerodynamic drag or xe2x80x9cwindagexe2x80x9d from heating the rim and causing potential failure as well as loss of efficiency. Some systems, usually operating at lower speeds, can use an operating chamber partially filled with a low density gas like hydrogen or helium instead of evacuated to reduce aerodynamic drag to a lesser extent. However, the generally accepted practice is to operate flywheel rotors with composite material flywheel rims in a vacuum of 10xe2x88x923 Torr pressure or lower.
Whichever aerodynamic loss prevention system is employed, the composite rim will operate in a closed chamber. Although some flywheel systems maintain the vacuum in the chamber through the use of an internal getter or external vacuum pump, it is very desirable to eliminate these devices or at the least to reduce the gas loads that they must accommodate. This makes the vacuum system much simpler and less costly.
When composite material rims such as but not limited to filament wound carbon fiber/epoxy or glass fiber/epoxy are placed in a vacuum, moisture and other gases are released over time. These gases contaminate the vacuum in the chamber and cause the pressure to increase unacceptably. The rate of release of gases is strongly influenced by temperature and increases significantly at higher temperatures. A common preparation technique for composite rims before entering service in the vacuum chamber is to bake the rim at an elevated temperature while under vacuum for some duration of time until most of the outgassing of the composite is complete. Although this is an effective method for gas removal and ultimately the reduction of the outgas rate of the composite rim in the vacuum in service, it is costly, inconvenient and not always feasible. Vacuum baking the rim would need to be done just prior to final assembly and the baking could take hours to several days. Other components attached to the composite rim before final assembly sometimes can not withstand the vacuum baking process and the heat can possibly result in unacceptable stresses due to differences in thermal expansions of the rim and other components. The use of low outgassing resins to manufacture the composite flywheel rim is one way possible to reduce the outgas rate without vacuum baking. However, low outgas resins such as cyanate esters can cost as much as fifty times more than conventional epoxies. Because of the high cost of low outgas resins, they are not suitable for manufacture of typical low cost composite rims.
Thus, there has long been a need in the flywheel art for a low cost composite flywheel rim that has a low rate of outgassing so that it can operate efficiently and without maintenance for long periods in a vacuum chamber.
Accordingly, this invention provides a composite material flywheel rim having a reduced rate of outgassing in a vacuum. The invention provides a flywheel system having a composite flywheel rim that has a metal coating that reduces the outgassing rate from the resin in the composite rim.
The composite flywheel rotor of this invention includes an annular rim mounted on a hub for high speed rotation in an evacuated flywheel enclosure. The composite rim includes a fiber-wound annulus in an epoxy matrix. A smooth epoxy layer is applied to the rim and is cleaned or maintained clean in preparation for a metal coating on the rim. The rim may be baked in a vacuum furnace to drive off the volitiles and water vapor, and a thin metal coating is applied over the entire rim to retard outgassing from the resin in the flywheel composite rim. The thickness of the thin metal coating is preferably on the order of about 1000-250,000 angstroms, although thicknesses up to several thousanths of an inch would be suitable. The metal coating on the flywheel rim is preferably aluminum because aluminum adheres well to epoxy and is economical. The metal coating is deposited on the flywheel rim by physical vapor deposition and may be built up by electroplating after an initial PVD coating. A tough protective polymer such as epoxy, polyurethane or the like may be applied over the metal coating to protect the metal coating from mechanical damage during handling.